Japanese police said Friday they have arrested a man who threatened to bomb the U. S. embassy in Tokyo.

Police say Okinawa resident Mitsuyoshi Kamiya made phone calls threatening to bomb the embassy, as well as Camp Schwab, a U.S. military base on the southern Japanese island.

Local media report the 52-year-old Kamiya is also believed to be behind recent death threats against U.S. Ambassador Caroline Kennedy and other American diplomatic officials.

On Thursday, U.S. State Department spokeswoman Jen Psaki said Japanese police arrested a man in connection with the threats against both the embassy and the ambassador.

"We take any threats to U.S. diplomats and U.S. diplomatic facilities very seriously. We... have been working for several weeks with the Japanese government," she said.

Japanese authorities say Kamiya has admitted to the bomb threats, though a motive has not been discovered. He has not been charged with making the threats against Ambassador Kennedy.

Many Okinawans resent the presence of the tens of thousands of U.S. troops stationed on the island, citing troop behavior, noise pollution and the dangers of an aircraft crash on the densely populated island.

The threats surfaced as U.S. first lady Michelle Obama was in Japan for a three-day visit aimed at promoting her global education initiative for girls and women.

The developments follow an attack earlier this month on the U.S. ambassador to South Korea, Mark Lippert. He needed 80 stitches to close a wound on the right side of his face following the attack, which was carried out by a North Korea sympathizer.